Catalytic cracking process

Abstract: A hydrocarbon stream is cracked in Plant A to produce a hot gaseous stream which is separated in a fractionator B. A gaseous stream containing hydrogen and C₁-C₄ hydrocarbons is taken from the fractionator B, is raised in pressure in compressor C and is cooled in a heat exchanger D. The cooled gaseous stream is separated in a demethaniser E, and an off-gas comprising hydrogen and C₁ to C₃ hydrocarbons is withdrawn from the demethaniser E. The off-gas is subjected to separation by pressure swing adsorption or temperature swing adsorption at an adsorption temperature of about 0 to about 250°C in a bed of adsorbent which selectively adsorbs ethylene and propylene, thereby adsorbing substantially all of the ethylene and propylene from the gas stream. The ethylene and/or propylene is recovered upon bed regeneration.

Description: This invention relates to the cracking of hydrocarbons and more particularly to the recovery of olefins from the off-gas from a catalytic cracking operation. The effluent from a hydrocarbon cracking unit contains a wide spectrum of hydrocarbons. To recover the hydrocarbons the effluent is cooled and subjected to a series of separation steps, such as condensation and distillation to recover the heavy and light liquid components. After removal of these components, the remaining light gas stream can be compressed and cooled, thereby condensing most of the remaining hydrocarbons from the stream. The noncondensable gas remaining after the light gas compression and condensation step, generally referred to as off-gas, is composed substantially of hydrogen and small amounts of C 1 to C 3 hydrocarbons, and perhaps some other gaseous components, such as nitrogen and carbon dioxide. The off-gas is usually sent to flare or used as fuel. To minimize the amount of hydrocarbons remaining in the off-gas, the light gas stream is compressed to as high a pressure and cooled to as low a temperature as is practicable. Consequently, the energy expended in cooling and compressing the condensable light gases is considerable. It is desirable to reduce the overall cost of recovering cracked hydrocarbon products and maximize the amount of valuable C 2 and C 3 alkenes recovered from the hydrocarbon cracking unit off-gas. This objective could be attained if an efficient and cost effective method of recovering lower alkenes from gas streams were available. The present invention provides an alkene adsorption method which reduces the energy requirements of hydrocarbon cracking processes and provides substantially complete recovery of the lower alkenes contained in cracking unit off-gas. According to the invention, a process for the recovery of alkene selected from ethylene, propylene and mixtures of these from a cracked hydrocarbon stream comprising the steps: (a) separating a gaseous stream from the cracked hydrocarbon product; (b) cooling the gaseous stream, thereby producing a condensed hydrocarbon stream and a gas stream comprising hydrogen and methane and containing amounts of alkene and alkane, the alkane being selected from ethane, propane and mixtures of these; (c) subjecting said gas stream to a cyclic adsorption process in a bed of adsorbent which selectively adsorbs alkenes, thereby producing a nonadsorbed hydrogen- and alkane-enriched component and an adsorbed alkene-enriched component; and (d) desorbing said alkene-enriched component from said adsorbent; wherein the adsorption step of the cyclic adsorption process is conducted at a temperature of up to 250°C. The adsorption step is typically carried out at a temperature in the range of about 0°C to about 250°C, and is preferably carried out at a temperature above about 50°C. The adsorption step is generally carried out at an absolute pressure in the range of about 0.2 to 100 bar, and is preferably carried out carried out at an absolute pressure of about 1 to 50 bar. In a preferred embodiment of the invention, the adsorbent is a type A zeolite, and in the most preferred embodiment, it is type 4A zeolite. When the adsorption process is PSA, the pressure during the regeneration step is reduced, usually to an absolute pressure in the range of about 100 to about 5000 millibar, and preferably to an absolute pressure in the range of about 100 to about 2000 millibar. When the adsorption process is TSA, the bed temperature is usually raised during bed regeneration to a value in the range of about 100 to about 350°C, and is preferably raised to a value in the range of about 150 to 300°C. In other preferred embodiments of the invention the adsorption bed regeneration step is effected by vacuum means or by purging the bed with an inert gas, the nonadsorbed gas product from the adsorption system or the adsorbed product gas from the adsorption system, or by combinations of vacuum and purge regeneration; and bed repressurization is at least partly effected using the alkene-enriched desorbed gas from the adsorption system. The drawing illustrates, in a block diagram, a system for cracking hydrocarbons in accordance with a principal embodiment of the present invention. In the accompanying drawing auxiliary equipment not necessary for an understanding of the invention, including compressors, heat exchangers and valves, has been omitted to simplify discussion of the invention. In the drawing, A is a hydrocarbon cracking plant, B is a fractionator, C is a gas compressor, D is heat exchanger, F is a demethanizer or a flash chamber and F is an adsorbent-based gas separation system. Plant A may be any hydrocarbon cracking system typically used in petroleum refining operations. The particular cracking method employed in the process of the invention forms no part of the invention and any of the commonly used thermal and catalytic cracking processes can be used in the practice of the invention. Cracking unit A is typically equipped on its inlet end with hydrocarbon feed line 2 and its cracked gas outlet is connected to the inlet of fractionator B via line 4. Fractionator B is a conventional fractionating column designed to produce an overhead stream comprising C 4 and lighter hydrocarbons, a side stream comprising of C 5 and heavier liquid hydrocarbons and a bottoms stream comprising heavy residual components. The overhead stream, the side stream and residual product stream are discharged from column B through lines 6, 8 and 10, respectively. Line 10 is connected to the inlet of unit A through line 12. Line 6 joins the overhead outlet of column B with the inlet of unit E. Compressor C and cooler D are located in line 6. Compressor C and cooler D are any typical gas compressor and heat exchanger usable for compressing and cooling hydrocarbon gases. Unit E is any conventional flash chamber or fractionating column, and it is designed to separate the noncondensable off-gas from the condensable light hydrocarbon components contained in the feed stream to this unit. The condensed light hydrocarbons are discharged from unit E through line 14. Line 16 connects the off-gas outlet of unit E to the inlet of separator F. Separator F is an adsorption system whose principal function is to separate the alkenes contained in the off-gas from unit E (mainly ethylene or propylene) from the other gases contained in this stream. This unit is typically a pressure swing adsorption or temperature swing adsorption system, generally comprising two or more stationary beds arranged in parallel and adapted to be operated in a cyclic process comprising adsorption and desorption. In such systems the beds are cycled out of phase to assure a pseudo-continuous flow of alkene-enriched gas from the adsorption system. The beds of separator F are packed with an adsorbent which selectively adsorbs alkenes from a gas mixture containing the alkenes and one or more alkanes. In general, the adsorbent may be alumina, silica, zeolites, carbon molecular sieves, etc. Typical adsorbents include alumina, silica gel, carbon molecular sieves, zeolites, such as type A and type X zeolite, type Y zeolite, etc. The preferred adsorbents are type A zeolites, and the most preferred adsorbent is type 4A zeolite. Type 4A zeolite, i.e. the sodium form of type A zeolite, has an apparent pore size of about 3.6 to 4 Angstrom units. This adsorbent provides enhanced selectivity and capacity in adsorbing ethylene from ethylene-ethane mixtures and propylene from propylene-propane mixtures at elevated temperatures. This adsorbent is most effective for use in the invention when it is substantially unmodified, i.e. when it has only sodium ions as its exchangeable cations. However, certain properties of the adsorbent, such as thermal and light stability, may be improved by partly exchanging some of the sodium ions with other cations. Accordingly. it is within the scope of the preferred embodiment of the invention to use a type 4A zeolite in which some of the sodium ions attached to the adsorbent are replaced with other metal ions, provided that the percentage of ions exchanged is not so great that the adsorbent loses its type 4A character. Among the properties that define type 4A character are the ability of the adsorbent to selectively adsorb ethylene from ethylene-ethane mixtures and propylene from propylene-propane gas mixtures at elevated temperatures, and to accomplish this result without causing significant oligomerization or polymerization of the alkenes present in the mixtures. In general, it has been determined that up to about 25 percent (on an equivalent basis) of the sodium ions in 4A zeolite can be replaced by ion exchange with other cations without divesting the adsorbent of its type 4A character. Cations that may be ion exchanged with the 4A zeolite used in the alkene-alkane separation include, among others, potassium, calcium, magnesium, strontium, zinc, cobalt, silver, copper, manganese, cadmium, aluminum, cerium, etc. When exchanging other cations for sodium ions it is preferred that less than about 10 percent of the sodium ions (on an equivalent basis) be replaced with such other cations. The replacement of sodium ions may modify the properties of the adsorbent. For example, substituting some of the sodium ions with other cations may improve the stability of the adsorbent. Another class of preferred adsorbents are those which contain certain oxidisable metal cations, such as copper-containing adsorbents, which possess enhanced adsorptive capacity and selectivity with respect to the preferential adsorption of alkenes from gaseous alkene-alkane mixtures. Suitable adsorbent substrates for manufacturing copper-modified adsorbents include silica gel, and zeolite molecular sieves, such as zeolite type 4A, zeolite type 5A, zeolite type X and zeolite type Y. The manufacture and use of copper-modified adsorbents and examples of suitable copper-containing adsorbents are set forth in US Patent No 4,917,711. Separator F is provided with waste gas discharge line 18, purge gas line 22 and alkene discharge line 20, which, in the embodiment illustrated in the drawing, is connected to condensed light hydrocarbon discharge line 14. Purged gas recycle line 24 connects line 20 to the inlet to separator F. According to the process of the invention practiced in the system illustrated in the drawing, a hydrocarbon cracker feed stream, such as gas oil, is introduced into cracking unit A. The hydrocarbon feed is typically cracked into a hot gaseous product comprising mixed hydrocarbons, e.g. hydrocarbons having up to about 12 carbon atoms, and a heavy hydrocarbon residual product. The hot gaseous product leaves unit A and is next separated in fractionator B into a heavy residual stream, which is removed through line 10 and discharged from the system or recycled to unit A through line 12; an intermediate hydrocarbon stream mainly composed of liquid hydrocarbons having 5 or more carbon atoms, which is removed through line 8; and a light hydrocarbon gas stream composed substantially of hydrogen, hydrocarbons having up to 4 carbon atoms, and, if present in the original feed, nitrogen, which leaves column B via line 6. The light hydrocarbon gas stream passing through line 6 is compressed in unit C to the desired pressure, cooled in heat exchanger D to the temperature at which most of the C 2 to C 4 hydrocarbons in the stream are condensed and introduced into unit E. A product stream composed of the readily condensable components of the feed to unit E is removed from this unit through line 14 and sent to downstream processing units for further hydrocarbon separation. A gas stream comprised predominantly of hydrogen and C 1 to C 3 hydrocarbons is discharged from unit E through line 16 and is introduced into separator F. As the off-gas passes through the adsorption beds of separator F the alkene components of the stream are adsorbed onto the adsorbent while the hydrogen and alkanes (and any nitrogen present) in the gas stream pass through the adsorbent and exit separator F through line 18 as nonadsorbed gas. Separator F is preferably operated in a manner which results in the adsorption of substantially all of the alkene and rejection of most of the hydrogen and alkane present in the feed to this unit. The temperature at which the adsorption step is carried out depends upon a number of factors, such as the particular adsorbent being used, e.g. unmodified 4A zeolite, a particular metal-exchanged 4A zeolite or another adsorbent which selectively adsorbs alkenes from alkene-alkane mixtures, and the pressure at which the adsorption is carried out. In general, the adsorption step is carried out at a minimum temperature of about 0°C and is preferably carried out at a minimum temperature of about 50°C and is most preferably carried out at a temperature of at least about 70°C. The upper temperature limit at which the adsorption step in unit A is carried out is determined mostly by economics. In general the adsorption step can be carried out at a temperature below the temperature at which the alkene undergoes chemical reaction, such as polymerization. The upper adsorption temperature limit is about 250°C. When unmodified 4A zeolite is used as the adsorbent the reaction is generally carried out at or below 200°C, and is preferably carried out at a temperature at or below 170°C. Oxidisable metal-containing adsorbents, such as copper modified adsorbents, are particularly effective at temperatures above about 100°C, for example at temperatures between about 100°C and 250°C. They are preferably used at temperatures in the range of about 110 to 200°C, and most preferably at temperatures in the range of about 125 to about 175°C. The pressures at which the adsorption step is carried out generally ranges from about 0.2 to about 100 bar, and preferably from about 1 to 50 bar for pressure swing adsorption cycles, and is usually about atmospheric or above for temperature swing adsorption cycles. When the adsorption process is PSA the regeneration step is generally carried out a temperature in the neighbourhood of the temperature at which the adsorption step is carried out and at an absolute pressure lower than the adsorption pressure. The pressure during the regeneration step of PSA cycles is usually in the range of about 20 to about 5000 millibar, and preferably in the range of about 100 to about 2000 millibar. When the adsorption process is TSA, bed regeneration is carried out at a temperature higher than the adsorption temperature, usually in the range of about 100 to about 350°C, and preferably in the range of about 150 to 300°C. In the TSA embodiment, the pressure is generally the same during the adsorption and regeneration steps, and it is often preferred to conduct both steps at about atmospheric pressure or above. When a combination of PSA and TSA is used the temperature and pressure during the bed regeneration step are higher and lower, respectively, than they are during the adsorption step. When the adsorbed alkene front travelling through the vessel(s) of separator F in which the adsorption step is being carried out reaches the desired point in the vessel(s), the adsorption process in these vessel(s) is terminated and these vessel(s) enter the regeneration mode. During regeneration, the alkene-loaded vessel(s) are depressurized, if the adsorption cycle is pressure swing adsorption, or heated, if a temperature swing adsorption cycle is employed. As the regeneration proceeds, alkene-enriched gas is discharged from separator F through line 20. This stream can be combined with the light hydrocarbon stream in line 14, as illustrated in the drawing, or discharged from the system through line 26 for further processing. The method of regeneration of the adsorption beds depends upon the type of adsorption process employed. In the case of pressure swing adsorption, the regeneration phase generally includes a countercurrent depressurization step during which the beds are vented countercurrently until they attain the desired lower pressure. If desired the pressure in the beds may be reduced to subatmospheric pressure by means of a vacuum inducing device, such as a vacuum pump (not shown). In some cases, in addition to the countercurrent depressurization step(s), it may be desirable to purge the bed with an inert gas or one of the gas streams exiting separator F. In this event the purge step is usually initiated towards the end of the countercurrent depressurization step, or subsequent thereto. During the purge step, a nonadsorbable purge gas can be introduced into separator F via line 22 and passed countercurrently through the adsorbent beds, thereby forcing desorbed alkene out of out of separator F through line 20. The purge gas may be nonadsorbed product gas exiting separator F through line 18, or a nonadsorbable gas obtained from a different source, such as an inert permanent gas like nitrogen. In a preferred method of operation of the system of the drawing, the alkene desorbed from separator F during the countercurrent depressurization step(s) is discharged into line 14, and all or a portion of the purge gas and alkene desorbed from the bed during the purge step is recycled to separator F through line 24 for reprocessing. The advantage of this embodiment is that it permits the amount of purge gas that is transferred to line 14 to be minimized. The adsorption cycle may contain steps other than the fundamental steps of adsorption and regeneration. For example, it may be advantageous to depressurize the adsorption bed in multiple steps, with the first depressurization product being used to partially pressurize another bed in the adsorption system. This will further reduce the amount of gaseous impurities transferred to line 14. It may also be desirable to include a cocurrent purge step between the adsorption phase and the regeneration phase. The cocurrent purge is effected by terminating the flow of feed gas into separator F and passing high purity alkene cocurrently into the adsorption bed at adsorption pressure. This has the effect of forcing nonadsorbed gas in the void spaces in separator F toward the nonadsorbed gas outlet, thereby ensuring that the alkene produced during the countercurrent depressurization will be of high purity. The high purity alkene used for the cocurrent purge can be obtained from an intermediate storage facility (not shown) in line 20, when separator F comprises a single adsorber; or from another adsorber that is in the adsorption phase, when separator F comprises multiple adsorbers arranged in parallel and operated out of phase. It will be appreciated that it is within the scope of the present invention to utilize conventional equipment to monitor and automatically regulate the flow of gases within the system so that it can be fully automated to run continuously in an efficient manner. An important advantage of the invention is that it permits removal of valuable alkenes from a hydrocarbon cracking unit off-gas stream without also removing substantial amounts of the low value alkanes contained in the off-gas. It will be appreciated that a system that achieves enhanced selectivity, and hence increased overall recovery of alkenes from a cracking operation is highly beneficial. The invention is further illustrated by the following hypothetical example in which, unless otherwise indicated, parts, percentages and ratios are on a volume basis. The example illustrates the process of the invention as it applies to the catalytic cracking of a gas oil. EXAMPLE I A gaseous gas oil stream is processed in a fluid catalytic cracker containing a catalyst based on type Y zeolite and other active components at a temperature of about 400°C, thereby producing a gaseous product stream. The gaseous product is fractionated into a viscous bottoms product, which is combined with the gas oil feed to the catalytic cracking unit; a condensed mixed hydrocarbons side stream containing mostly C 5 and higher hydrocarbons, which is removed as a liquid product; and a gaseous overhead stream comprised mostly of C 4 and lighter hydrocarbons. The overhead stream is compressed to a pressure of 33 bar, cooled to a temperature of 15°C and introduced into a light hydrocarbon fractional distillation unit, wherein the overhead stream is split into a bottoms stream comprising most of the hydrocarbons and an overhead noncondensable gas stream having the concentration listed in the Table as stream 1. The noncondensable gas stream is subjected to a pressure swing adsorption process having a two minute cycle in an adsorption system comprised of a pair of adsorption vessels packed with type 4A zeolite. The adsorption vessels are arranged in parallel and operated out of phase. During the adsorption step the beds are maintained at a temperature of 100°C and an absolute pressure of 8 bar, and during bed regeneration the beds are depressurized to an absolute pressure of 1.2 bar. Desorbed and nonadsorbed gas streams having the compositions listed in the Table as streams 2 and 3, respectively, are obtained. COMPONENTS STREAM 1 STREAM 2 STREAM 3 lbmoles/hr lbmoles/hr lbmoles/hr hydrogen 178.8 17.9 160.9 methane 955.8 372.7 583.0 ethane 402.7 169.2 233.6 ethylene 209.6 167.7 41.9 propylene 248.7 156.7 92.0 propane 32.9 11.8 21.0 isobutane 2.0 0.0 2.1 1-butene 2.0 0.0 2.0 cis 2-butene 0.0 0.0 0.0 normal butane 2.0 0.0 2.0 isopentene 2.0 0.0 2.0 normale pentane 2.0 0.0 2.0 hexane 3.8 0.0 3.8 TOTAL 2,043.4 898.0 1,149.3

Claims: A process for the recovery of alkene selected from ethylene, propylene and mixtures of these from a cracked hydrocarbon stream comprising the steps: (a) separating a gaseous stream from the cracked hydrocarbon product; (b) cooling the gaseous stream, thereby producing a condensed hydrocarbon stream and a gas stream comprising hydrogen and methane and containing amounts of alkene and alkane, the alkane being selected from ethane, propane and mixtures of these; (c) subjecting said gas stream to a cyclic adsorption process in a bed of adsorbent which selectively adsorbs alkenes, thereby producing a nonadsorbed hydrogen- and alkane-enriched component and an adsorbed alkene-enriched component; and (d) desorbing said alkene-enriched component from said adsorbent; wherein the adsorption step of the cyclic adsorption process is conducted at a temperature of up to 250°C. A process according to claim 1, wherein said cyclic adsorption process is selected from pressure swing adsorption, temperature swing adsorption or a combination of these. A process according to claim 1 or claim 2, wherein the adsorption step is conducted at a temperature above 50°C. A process according to any one of the preceding claims, wherein the adsorbent is selected from alumina, type 4A zeolite, type 5A zeolite, type X zeolite, type Y zeolite and mixtures of these. A process according to any one of claims 1 to 3, wherein the adsorbent is a copper-modified zeolite type 4A, copper-modifier zeolite type 5A, copper-modified zeolite type X, or a copper modified zeolite type Y. A process according to claim 4, wherein said adsorbent is 4A zeolite that contains exchangeable cations other than sodium ions, but at a level insufficient to divest the adsorbent of its type 4A character. A process according to any one of the preceding claims, wherein said gas stream is separated from said condensed hydrocarbon stream by flashing, distillation or a combination of these. A process according to any one of the preceding claims, wherein the alkene-enriched component comprises ethylene and propylene and is combined with said condensed hydrocarbon stream.

Inventors: DAO LOC H, RAMACHANDRAN RAMAKRISHNAN, DAO, LOC H., RAMACHANDRAN, RAMAKRISHNAN
Applicants: BOC GROUP INC, THE BOC GROUP, INC.
Application year: 1994
Main classification: C10G70
IPC codes: C07C11, C10G5, C07C7, B01D53, C10G70